Both animals and human beings are able to have a wide range of stable movements in different environments with unpredictable obstacles. For this, a dynamic control of the stiffness of articulations to allow for their adaptation to these changes is crucial. Human articulations are groups of muscles that enable the control of both the stiffness of the articulation and its position, thus allowing for a wide variety of modes of locomotion and simultaneous adaptation to different ground with acceptable energy efforts. Currently, a large number of devices associated with locomotion such as prostheses, orthoses, exoskeletons and walking robots are trying to integrate devices that emulate the functioning of the natural muscles in order to obtain movements with better energy efficiency, adaptability, and to increase security in the human-robot interaction.
In the search for obtaining articulations, whose behaviour is similar to human ones and which enable taking advantage of the natural dynamics of the movements and the configuration of the articulation characteristics facing the different requirements of the motion, many designs and research are being oriented to the development of motorised articulations, which are capable of adapting to obstacles of an unknown environment and have energy storage capacity to reduce the energy efforts of locomotion.
Document U.S. Pat. No. 5,650,704 presents one of the first designs that incorporated an element providing both adaptation and shock absorption capacity to the actuating system. This actuator makes use of elastic elements connected in series to the power drive train and not only allows to attribute some of the characteristics of the natural muscle to an electric actuator, but also allows to control the force exerted on the load by the operator. The main disadvantage of this mechanism is that the stiffness of the system is fixed, so it is restricted to an energetically optimal operating speed and load.
Other designs have focused on the modification of the stiffness of the articulation with the purposes of energy optimisation and utilisation of the locomotion dynamics. The document “P. Cherelle, V. Grosu, P. Beyl, A. Mathys, R. Van Ham, M. Van Damme, B. Vanderborght and D. Lefeber. ‘The MACCEPA Actuation System as Torque Actuator in the Gait Rehabilitation Robot ALTACRO’ International Conference on Biomedical Robotics and Biomechatronics (2010)” presents the incorporation of a mechanically adjustable actuator to a rehabilitation exoskeleton, such an actuator allowing a safe interaction between the user and the machine and meeting the typical power requirements in rehabilitation of human locomotion. The document EP1726412A1 presents the base actuator used in the exoskeleton mentioned above, with the help of elastic elements and a motor responsible for variation in position and another one for the stiffness of the system, both characteristics being controlled independently. This design is aimed at rehabilitation and not strictly at a continuous variation of the parameters of the articulation to achieve greater locomotion efficiency.
The document “Sebastian Wolf and Gerd Hirzinger” A New Variable Stiffness Design: Matching Requirements of the Next Robot Generation 2008 IEEE International Conference on Robotics and Automation”, describes an articulation for a robotic arm, in which a parallel arrangement allows the variation of the stiffness of the articulation, making it adaptable. In addition to the motor that moves the articulation, an additional motor, responsible for the suitability of the system, is necessary, with the characteristic that both operate in antagonistic way, what represents an increase in energy consumption. Even so, by taking advantage of the movement dynamics and by the appropriate articulation stiffness variation, a reduction of the net energy for the operation cycle is obtained. The measurement of the force/torque in the articulation is obtained through the incorporation of an external torque sensor between the actuator and the load.
Document WO/2012/038931 A1 presents several configurations mainly developed by the Italian Institute of Technology (IIT). Like the above-mentioned designs, they use 2 motors to independently operate on the position of the articulation and its stiffness, with the novelty of requiring lower energy associated with the variation of stiffness because it does not operate in the same direction of action as the elements that move the articulation. The document “Amir Jafari, Nikos G. Tsagarakis and Darwin G. Caldwell. ‘AwAS-II: A New Actuator with Adjustable Stiffness based on the Novel Principle of Adaptable Pivot point and Variable Lever ratio’ 2011 IEEE International Conference on Robotics and Automation”, presents an actuator of this type, whose final implementation is expected to be in an exoskeleton knee articulation. However, the arrangement of the elements in this motorised articulation results in a system of still excessive dimensions for use in exoskeletons, mainly by the parallel arrangement of most of the parts of the device and the use of an external torque sensor between the actuator and the load.
All the actuated articulations that have controllable stiffness are characterised in that they typically include an element of measurement of the torque generated at the articulation, which is connected in parallel with the articulation actuating system. By making use of these elements traditionally connected in parallel with the structure, a considerable increase in the volume of the articulation is obtained, which is not at all desirable for use in exoskeletons for locomotion aid because what is of interest is to minimise its volume. This will be apparent in the designs of document WO/2012/038931 A1.